1. Field of the Invention
The embodiments discussed herein relate to a silicon carbide (SiC) semiconductor device and a method of manufacturing a silicon carbide semiconductor device using a semiconductor material such as silicon carbide.
2. Description of the Related Art
Recently, SiC has gained attention as a semiconductor material to replace silicon (hereinafter, Si). SiC has a bandgap of 3.25 eV for 4H-SiC, nearly 3 times the bandgap (1.12 eV) of Si, and therefore, can increase the upper limit operating temperature. Further, the dielectric breakdown field strength is 3.0 MV/cm for 4H-SiC, about 10 times greater than the dielectric breakdown field strength (0.25 MV/cm) of Si and therefore, the ON resistance, which exerts an effect by the third power of the reciprocal of the dielectric breakdown field strength, is reduced, enabling power loss in an always-ON state to be reduced. Further, since the thermal conductivity is 4.9 W/cmK for 4H-SiC, three times higher than the thermal conductivity (1.5 W/cmK) of Si, an advantage is obtained in that the size of a cooling apparatus having a high thermal cooling effect can be reduced. Since the saturated drift velocity is large at 2×107 cm/s, high-speed operation is also favorable. Because of these points, the application of SiC to power semiconductor elements (hereinafter, called power devices), high-frequency devices, high-temperature operating devices, and the like is expected.
At present, metal-oxide-semiconductor field-effect transistors (MOSFETs), pn diodes, Schottky diodes, and the like are being prototyped, and devices exceeding Si characteristics related the dielectric breakdown voltage and ON resistance (ON resistance=forward voltage/forward current at the time of energization) are appearing one after another. In the creation of these elements, a technique of controlling the conductivity type and carrier concentration in selected regions is necessary. Such methods include thermal diffusion methods and ion implantation methods. Since the impurity diffusion coefficient for SiC is extremely small, thermal diffusion methods, which are widely used in Si semiconductor devices, are difficult to apply to SiC. Therefore, with SiC, normally, an ion implantation method is used as a carrier concentration control technique. Implanted as an ion species, nitrogen (hereinafter, indicated as “N”) and phosphorus (hereinafter, indicated as “P”) are often used as an n-type and aluminum (hereinafter, indicated as “Al”) or boron (hereinafter, indicated as “B”) is often used as a p-type.
Large-capacity, high-voltage power devices have a vertical element structure where voltage is applied in a vertical direction of the element, i.e., between a front surface and a rear surface of the element and current is controlled between the front surface and the rear surface. Therefore, configuration is such that an electrode is on the front surface and on the rear surface of the semiconductor element. For example, in the case of a Schottky diode, configuration is such that a Schottky electrode is on the front surface (first main surface) of the element and an ohmic electrode is on the rear surface (second main surface). Further, in the case of a vertical MOSFET, configuration is such that a source electrode and a gate electrode substrate are on the front surface and a drain electrode that is an ohmic electrode is on the rear surface.
An important element in reducing the ON resistance of such a power device using SiC is a reduction of a contact resistance ρc of the ohmic contact. In particular, to reduce the ON resistance, a method of subdividing and arranging in a high density on the SiC substrate, a main electrode region of the power device is employed. In reducing the ON resistance of a micro-sized power device, obtaining a low contact resistance ρc in a minute opening becomes extremely important.
As a method of forming the above ohmic electrode in a silicon carbide semiconductor device constituted by a SiC substrate, a method of performing a silicide process involving heat treatment at about 1000 degrees C. to form a silicide film on the SiC substrate, after an electrode film of Ni or Ti, etc. is vapor deposited on the SiC substrate is typically performed to obtain an ohmic electrode having a low resistance (small potential barrier) contact with both an n-type SiC and a p-type SiC (for example, refer to “Fundamentals and Applications of SiC elements”, Ohmsha, Ltd. p. 112).